The light emitting diode (LED) is a fundamental component for electronic lighting technology. While a voltage is applied between the two electrode terminals of LED, which induces an electric current to pass through a p-n junction thereof, the LED will emit lights.
The LED is advantageous for its long duty life, fast responding, high reliability, high electro-optic transforming efficiency and low consumption of electricity. Moreover, the miniaturization of the LED makes it suitable for mass production and makes it meet the requirements and trends for the technical development.
LEDs emitting red, green, blue, infrared and ultraviolet lights have been marketed since early days, while the fabrication for the LED which emits visible lights is increasingly improved presently. Therefore, LEDs are further popularized and applied in various applications including the illumination and the communication.
Typically the LED has a layered structure consisting of a substrate, a buffer layer, an n-type layer, a p-type layer and an active layer, wherein a principal part of the LED is the active layer where the light is generated as a result of recombination process of electrons and holes injected thereinto. Such a layered structure as well as the light emitting semiconductor device containing the layered structure is disclosed in the U.S. Pat. No. 6,233,265. By means of growing light emitting device heterostructures over a thick InGaN layer, a stable InGaN structure is formed so as to avoid the lattice mismatch therefore and permit the realization of an unsegregated, high-indium-content InGaN alloy active region for brighter and more spectrally pure emission of lights.
Please refer to FIG. 1, which shows the layered structure of the LED disclosed in this patent. The LED 100 includes a single-crystal substrate 105, a first buffer layer 110 and a second buffer layer 112, a thick InGaN layer 120 which is n-type doped, an InGaN active layer 130 formed thereon, a first group III-V nitride layer 140 and a second group III-V nitride layer 150 formed thereon, wherein the first group III-V nitride layer 140 and the second group III-V nitride layer 150 are both p-type doped. Moreover, on the second group III-V nitride layer 150 and the exposed thick InGaN layer 120 are a p-electrode 160 and an n-electrode 170, respectively.
According to the specification of this patent, the two buffer layers 110 and 112 may have different alloy concentrations or be deposited under different conditions to promote the smooth film growth and to accommodate the large lattice mismatch therefore. Furthermore, since the first group III-V nitride layer 140 typically has a bandgap energy higher than that of the second group III-V nitride layer 150, so that the injected electrons are confined to the active region. As a result, the InGaN alloy phase separation is minimized and the compositional uniformity of InGaN layers is improved, so that the spectral emission of blue, green, and even red LEDs becomes more pure thereby.
The conventional LED chip is cut from this structure and packed in epoxy with a refractive index of 1.5 which is less than those of the layers in the structure. Since the LED structure has a planar shape and consists of the layers which have refractive indices higher than 1.5, a planar waveguide is easily formed therein which may capture the lights emitted at high incident angles. Such a wave guiding effect will seriously suppress the light extraction efficiency of LED and is related to the planar technology for the LED structure growth from vapor phase.
A method and an LED structure for improving the light extraction efficiency are disclosed in the U.S. Pat. No. 6,614,060. Please refer to FIG. 2, which schematically shows the LED structure according to the disclosure of this patent. The LED 200 typically includes a substrate 210, a buffer layer 220, an n-type contact layer 230, an active layer 240, a p-type contact layer 250 and an n-electrode as well as a p-electrode which are not shown in this figure. In order to further reduce the electron current leakage, a blocking layer 245 is configured between the active layer 240 and the p-type contact layer 250. Besides, the active layer 240 may be a layer sequence which has two well systems (WW, QW) with charge asymmetric resonance tunneling. One of the well systems is a wide well (WW) 241 and the other is an active quantum well (QW) 243, which are both made of InGaN. The wide well (WW) 241 and the active quantum well (QW) 243 are coupled via a resonance tunneling barrier 242 which is transparent for electrons and blocking for holes.
Accordingly, the capture efficiency for electrons is increased due to the direct tunneling of electrons from the WW 241 into the QW 243, so that the electrons current leakage into the p-type contact layer 250 is suppressed and the light extraction efficiency of LED 200 is further enhanced. However, such a design is disadvantageous for that a small number of structural defects existing in the mentioned layer sequence will seriously damage the light extraction efficiency of LED, and the wavelength of light generated by the QW is fixed so that a white light is unable to be generated therethrough.
Based on the above, it is apparent that the light extraction efficiency of a conventional light emitting semiconductor device is dominated and limited by the planar waveguide effect resulting from the planar technology for the light emitting semiconductor device and by the electrons current leakage in the active layer thereof.
For overcoming the mentioned drawbacks in this art, a novel structure of the light emitting semiconductor device is provided in the present invention. In comparison with the conventional ones, the provided light emitting semiconductor device is macroscopically planar but is not planar at a micro-scale or a nano-scale, so as to destroy the parasitic wave guiding effect and reduce the leakage currents thereof. Hence the light extraction efficiency of light emitting semiconductor device is increasingly enhanced thereby. Furthermore, the present invention also allows the direct generation for white light via a simplified method.